Pallial sinus: An
indentation in the posterior part of the pallial line where
the siphons can be retracted.

Dimyrian: A valve having two
adductor muscle scars; one anterior and one posterior.

Isomyrian: A dimyrian shell
where two adductor scars generally equal in size.

Anisomyrian: Dimyrian shell
where the two adductors are of unequal size; usually the
posterior scar is the larger of the two.

Monomyrian: A shell having
only one adductor scar; which is usually a centrally
positioned posterior adductor scar.

Figure 4 -
Interior
Shell Markings

Feeding

Most bivalves are filter feeders,
trapping suspended food particles as water passes through
their gills. Only two groups, the nuculoids and cryptodonts,
actively feed on organic material within the sediment and
are thus true deposit feeders. Taxodont dentition is
characteristic of deposit feeders.

Relations
to Substrate

Bivalves have a variety of
morphologic features that can be related to their particular
life habit or mode of attachment to the substrate. We will
examine several.

Boring: Shells are
usually thick, equivalved, and cylindrical in cross
section. Some forms are moderately ornamented with ridges
and stout spines
whereas others such as the "ship worms" are tubular in
form.

Semi
Infaunal

Byssally attached
(endobyssate). Similar to many epifaunal byssate
forms (see below), yet maximum shell width (inflation) is
at mid-line of shell cross-section. Some forms can be
elongated and fan-like with a reduced anterior area.
Examples include pen shells,
and the mussel-like modiolids,
and some ark shells. The depth to which the bivalves are
partially buried can often be deduced by looking for
encrusting organisms that may have attached themselves
above the sediment-water interface.

Epifaunal

Byssally attached
(epibyssate). Shells can be either equivalved or
inequivalved depending on their orientation to substrate
during life. Usually, all epibyssate forms have a reduced
anterior region. Some groups, such as the blue mussels,
are similar to endobyssate forms except the maximum
inflation is below the mid-line of the valves
cross-section. Other forms may have a byssal notch and/or
a well defined auricle,
or, as in the case of some arks, have a gape along the
ventral margin.

Reclining. Shells are
commonly inequivalved with a larger lower (usually the
left) valve which is more inflated or convex while the
upper valve may be planar. Some also exhibit spines,
especially on the lower valve, to aid in stabilization in
soft substrates in a manner similar to some brachiopods.
Many have a small attachment area at beak where earliest
growth stages were cemented.
The giant clam Tridacna,
who has photosymbionts similar to hermatypic
scleractinian corals, is a recliner even though it had a
functional byssus during its earliest juvenile
stages.

Swimming. Shells are
usually equilateral but not equivalved. The lower
(usually the left) valve is usually slightly larger.
Swimming forms are typified by having a greater umbonal
angle (greater than 105°) than similar-looking
epibyssate forms. Furthermore, swimming forms typically
have a single (monomyrian), large, centrally located
adductor muscle.

Cementing. Shells are
commonly inequivalved with the lower (usually left) valve
assuming the form of the object to which it is cementing,
a condition called xenomorphism. In such cases, both
valves are usually highly variable in shape, as in the
common oysters
and other forms as well. Some groups such as the
Cretaceous rudists
could reach very large sizes and were able to form reefs
mimicking corals in both morphology and ecology.